The present invention relates to a deflection control type ink jet recording or printing apparatus which ejects pressurized ink, imparts vibration to the ejected ink to regularly form droplets of ink, selectively develops a charging electric field during the formation of ink droplets on the basis of image signals, charges ink droplets with this electric field, and deflects the charged ink droplets with a deflecting electric field. More particularly, the present invention is concerned with a multi-nozzle, deflection control type ink jet recording apparatus having a number of ejection ports arranged linearly.
An ink jet recording apparatus of the type described is available in three different systems: a two-value deflection control system, a multi-value deflection control system and a combined system of the two. In the two-value system, droplets of ink to record information are charged (or charged to a high level) while non-recording ink droplets are left non-charged (or charged to a low level or an opposite polarity) so that the recording droplets are deflected to a large extent by a deflecting electric field and caused to impinge on a copy sheet with the non-recording droplets captured by a gutter. Conversely, the non-recording droplets may be deflected to a large extent and captured by a gutter. With this type of system, one ink jet nozzle is provided for recording one picture element. Concerning the multi-value system, one nozzle is provided for recording three or more picture elements (e.g. 40 dots along a width of 5 mm in the case of 8 dots/mm for instance) and ink droplets are charged to three or more levels (40 levels for example) and deflected along three or more paths (40 paths for example). In the combined system, charging is performed in the same way as in the multi-value system while charged recording droplets are deflected by a deflecting electric field extending in the Y-axis direction to clear a gutter and then deflected by a deflecting electric field in the X-axis direction in accordance with the charge level. This records information with positional differences along the X-axis direction on a recording sheet.
Meanwhile, ink ejected from a nozzle can be vibrated by any one of three different methods: a method which applies pressure vibration to ink itself, a method for applying vibration to a nozzle plate which defines an ejection port in the ink ejecting direction, and a method for causing the head to bodily vibrate in the ink ejecting direction. The first method is achievable with an arrangement wherein a nozzle plate having one ejection port is bonded to the front end of a cylindrical electrostrictive vibrator with the other end being communicated with a pressurized ink supply box or chamber. Another possible arrangement for the first method utilizes a slitted port formed through the front wall of a pressurized ink supply box, a nozzle plate having numerous ejection ports and being bonded to the ink supply box in such a manner as to cover the slitted port, and one or more flat electrostrictive vibrators mounted to one side wall of the ink supply box, the individual elements cooperating to impart a vibrating pressure to ink inside the box. For the second method, a porous nozzle plate may be secured through an elastic material to a pressurized ink supply box and caused to vibrate by an electrostrictive vibrator. For the third method, the head itself may be vibrated by a motor, a solenoid device, an electrostrictive vibrator or the like.
In a deflection control type ink jet recording apparatus employing any one of such known systems, the distance between the nozzle plate and recording sheet is relatively long. Therefore, the ink pressure is set at a high level so that an ink particle safely reaches the recording sheet while describing a stable path despite the actions of the charging electric field and deflecting electric field. Meanwhile, there must be determined a viscosity of the ink and a velocity of the ejected ink in order that ink droplets of a selected size are formed regularly and deposited with predetermined amounts of charge at a precise timing. When the ink pressure is at a given desired level, the viscosity and velocity are dependent on the temperature of the ink. Accordingly, the ink temperature should preferably be maintained constant to always promote stable recording against possible changes in the ambient or room temperature. For this purpose, ink is heated to a selected level in consideration of the level of the room temperature.
In this way, ink is pressurized and heated immediately before a recording operation begins. After the pressure and temperature of ink have reached and stabilized within predetermined ranges, charging voltage (pulse) and phase relative to a droplet separating phase, or vice versa, are set (phase search) and then a recording operation is started.
Pressurizing and heating ink just before a start of recording gives rise to a problem, however. At the instant communication is established between a source of pressurized ink supply and an ejection head to start the supply of ink under pressure to the ejection head, tiny droplets of ink are discharged as a spray from individual ejection ports and scattered therearound contaminating the charging electrode and various elements adjacent to the charging electrode. Moreover, such tiny droplets even though non-charged may happen to impinge on and contaminate a recording sheet as a result of disturbance of the predetermined paths. The cause of this phenomenon is as follows. While the apparatus is inoperative with the head disconnected from the pressurized ink source, atmospheric pressure prevails in an ink chamber communicating with ink ejection ports and this admits atmospheric air into the ejection ports and ink chamber in combination with the outflow of ink through the ejection ports. Additionally, while ink in the ink chamber is static, air contained in the ink is separated therefrom and floats upwardly to an upper part of the ink chamber. Air also stays in the ejection ports communicated with the ink chamber. When heated and pressurized ink is supplied to the ink chamber of the thus conditioned head, air inside the chamber is partly discharged through the ejection ports together with the ink forming an ink spray outside the head. This occurs repeatedly in a continuous and intermittent manner until air inside the ink chamber is sufficiently dissipated. As the temperature of the ink within the ink chamber rises, air inside the ink chamber expands until, at a certain time, the ink is sprayed out all of a sudden. Hence, a sufficiently long period of time is needed before a phase search or a start of recording can begin.
Ink ejection heads of the type described generally employ one of three different systems. A head acording to a first system is supplied with pressurized ink into its ink chamber and ejects the ink continuously due to the ink pressure (e.g. a deflection control type ink jet head). A second system known as an ink-on-demand system supplies an ink chamber of a head with non-pressurized ink and causes the head to eject the ink intermittently by applying a pulsating pressure produced by pulsating drive of an electrostrictive vibrator to the ink. A third system causes a head to eject ink by applying a sucking electric field to ink present at its ejection port. In any of such systems the ink ejection port takes the form of a minute opening (circular or rectangular) or a slit. Where a head thus constructed is used for printing operation with ink, the ink ejection characteristics such as the size of ink droplets and ejection velocity vary with the viscosity of ink which in turn varies with the temperature if the quality of the ink is the same. Therefore, the temperature of the ink is generally maintained constant so that the viscosity may remain constant. This is achievable by heating the ink to a certain level of temperature.
While the minute ink ejection port is formed through a metal or non-metal plate, the port must be formed with accuracy because its shape is another factor which affects the ink ejection characteristics of a head. Etching is a preferred method of forming such an ink ejection head with accuracy. Materials which can be etched to prepare nozzle plates include thin plates of silicone, ruby, sapphire and like monocrystalline materials and thin plates of monocrystalline metals. Conventionally, a nozzle plate in the form of a usual thin plate of metal or monocrystalline silicone formed with an ink ejection port is securely connected to a member having a liquid chamber therein by welding, brazing or cementing. The member with a liquid chamber is usually formed of stainless steel or like metal which is corrosion-resistant against ink. In such a conventional ink ejection head, it is difficult to machine that surface of the liquid chamber member to be engaged by the nozzle plate into a flat surface with precision. The nozzle plate therefore tends to be deformed or broken when connected to the liquid chamber member. Breakage is particularly liable to occur when the nozzle plate comprises a multi-nozzle, mirror-finished thin plate of monocrystalline silicone or like non-metallic material having a plurality of ink ejection ports. Since the nozzle plate bonded to the liquid chamber member cannot maintain its flat position with accuracy, the individual ejection ports of the multi-nozzle are oriented in various ejection directions which prevents images of good quality from being reproduced with the ink. Furthermore, the temperature of the ink differs a great deal from the operative condition of the apparatus to the inoperative condition because the ink is heated during the recording operation. Such a temperature difference causes the liquid chamber member and nozzle plate to expand and contract differently from each other due to their different coefficients of thermal expansion, resulting also in the deformation and/or breakage of the nozzle plate.